Newsgroups: rec.audio.tech
Subject: Re: Noise/Hum Problem
References: 
From: Tomi Holger Engdahl 
Date: 10 Jan 2000 15:55:12 +0200
Message-ID: 
Organization: Helsinki University of Technology, Finland
Lines: 50
X-Newsreader: Gnus v5.6.45/Emacs 19.34
"IJ"  writes:

> I want to play my collection of downloaded mp3 music through my stereo
> system, so I got a cable from Radio Shack that on one end connects into the
> line out on my sound card, and on the other end has 2 RCA jacks that connect
> to the back of my receiver. The music plays, but there's a hum no matter
> which ports I plug them into. For example: if I plug the RCA jacks into the
> outlets on the back of the receiver for the CD player, there is a hum.
> However, when I plug the CD player into those outlets, no problem. I tried
> turning off the monitor on the computer thinking that perhaps it was
> contributing to the problem, but this made no difference. Any suggestions?

You have quite proably a ground loop problem caused by the fact that there
is a power ground (through computer power supply) and antenna ground
(through receiver or VCR) to your audio system. This is not a good
thing and causes uusally humming problems. You problem goes away
if you disconnect the antenna from your system or install a
galvanic isolator to your antenna cable.

Other ways of getting ground loops like is is not easy.
The only acceptable method for testing is to remove and disconnect
everything. Then reinstall the equipments and connection piece by
piece. Stop when the hum becomes audible, and fix the problem in the
last piece of equipment that was installed. 

If you have equipments connected to many power outlets, then try to
connect them all to one grounded extension cord and then plug this
cord to one grounded outlet. This solves usually many problems,
because this creates star-like grounding scheme for those equipments. 

But the following star grounding scheme is effective if the equipments
do not have any direct connection to other equipment grounded
elsewhere (transformer isolated connections are ok). 

For more information take a look my documents on the topic at
http://www.hut.fi/Misc/Electronics/docs/groundloop/
 
-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/